The present invention is directed to a masterbatch composition containing polymethylmethacrylate (PMMA) pearls with improved adhesion and the corresponding masterbatch obtained thereof which, when used for the production of mono- or multilayer polyolefin films, leads to polyolefin films with improved integrity and stability properties.
In the development of packaging materials, especially for transparent packaging, mono- or multilayer polyolefin films have gained more and more importance because they are high in gloss, scratch-proof and durably transparent. Depending on the use these mono- or multilayer polyolefin films may be further metallized on their exterior layer, like for example metallized bi-oriented polypropylene (BOPP) films which are used in the packaging of for instance potatochips because of their barrier properties. Therefore, multilayer polyolefin films represent an interesting alternative for cellophane as packaging material.
However, these mono- or multilayer polyolefin films may cause problems when being used in high-speed packaging and/or processing machines because of their high film-to-film and film-to-metal coefficient of friction. This high coefficient of friction causes the films to adhere to each other and to develop a tendency of becoming sticky when surface pressure is applied.
To prevent these disadvantageous effects, it is known from the state of the art to use films in which antiblocking agents are incorporated. In this respect, different tendencies can be observed. On the one hand, anorganic antiblocking agents like silica, talc, calcium carbonate, and silicates were used. However, the use of such antiblocking agents leads to a deterioration in optical properties of the obtained films and, additionally, because of an insufficient binding of said antiblocking agents to the polyalkylene matrix, to an extraction of those particles when slitting the film, causing their distribution over the film and the processing machine.
To overcome these problems, U.S. Pat. No. 5,969,007 describes the use of low density or high density polypropylene or polyethylene grafted with maleic anhydride or an ethylene-acrylic acid-copolymer as a surface-modifying reagent which is supposed to function as an adhesive promoter between the polyalkylene matrix and the silicate anti-blocking particles when producing masterbatches thereof.
To further improve the optical properties of the polyolefin films on the other hand, there has been a tendency to use organic or at least organic moieties containing slip agents, like polydialkylsiloxane or fatty amides. However, those slip agents exhibit migratory properties which as well deteriorates the optical properties. Thus, WO 03/004269 presents the use of polymethylmethacrylate (PMMA) pearls in the exterior layer, which do not migrate.
Since such films still show disadvantages with regard to friction, transparency, and gloss, U.S. Pat. No. 5,077,129 discloses the use of a copolymer or mixtures of polymerized isobutyl methacrylate and methyl ethyl methacrylate.
However, because of a lack of binding within the polymeric matrix material, even the combination of these two methacrylate compounds leads to the above already mentioned problem known in the industry as “scuffing” which is the extraction of the particles out of the polymer surface under high shear developed for instance during mono-orientation of said films or during packaging processes on high speed packaging lines.